1. Field of the Invention
The present invention relates to a cascade scanning optical system having a plurality of laser scanning optical systems which are arranged along the main scanning direction and controlled to operate in synchronization with each other so as to realize a wide scanning line.
2. Description of the Related Art
A cascade scanning optical system having a plurality of laser scanning optical systems arranged along the main scanning direction to realize a wide scanning line is known. Such a scanning optical system is disclosed in Japanese Laid-Open Patent Publication No. 61-11720, published on Jan. 20, 1986. This publication discloses a cascade scanning optical system having a pair of laser scanning optical systems each having a laser beam emitter, a polygon mirror serving as a deflecting device, an f.theta. lens, etc The pair of laser scanning optical systems are synchronously driven to emit respective scanning laser beams to a photoconductive surface (scanning surface) of a photoconductive drum on a common line thereon extending in parallel to the axial direction of the photoconductive drum. The pair of scanning laser beams respectively scan two adjacent ranges of the common line on the photoconductive surface so as to scan the photoconductive surface of the photoconductive drum in the main scanning direction in a wide range.
There is a fundamental problem to be overcome in such a cascade scanning optical system. Namely, how can a scanning line, made on the photoconductive drum by the scanning laser beam emitted from one laser scanning optical system of the cascade scanning optical system, be accurately combined with another scanning line, made on the photoconductive drum by the scanning laser beam emitted from another laser scanning optical system of the cascade scanning optical system, so as not to be apart from each other or overlap each other in either the main scanning direction or the sub-scanning direction, i.e., so as to form a straight and continuous scanning line using a combination of the separate scanning lines.
In the case where each laser scanning optical system of the cascade scanning optical system is designed as a non-telecentric system through which the incident angle of a scanning laser beam relative to the photoconductive surface of the photoconductive drum varies in accordance with a variation in the position of a scanning spot of the scanning laser beam on the photoconductive surface in the main scanning direction, a scanning line made through one laser scanning optical system will not be precisely combined with another scanning line made through another laser scanning optical system on the photoconductive surface of the photoconductive drum, i.e., those scanning lines will be apart from each other or overlap each other by a certain amount in the main scanning direction if the photoconductive surface deviates from its original position even by a slight amount.
In the case where each laser scanning optical system of the cascade scanning optical system is designed as a telecentric system through which the scanning laser beam of each laser scanning optical system is always incident on the photoconductive surface in a direction perpendicular to an axial direction of the photoconductive drum, specifically in a direction orthogonal to generatrices of the photoconductive drum, the aforementioned problem in the case of the non-telecentric system will not occur, but it is necessary for each laser scanning optical system to be arranged at different positions in the sub-scanning direction to differentiate the angles of respective laser beams relative to the photoconductive surface, in order to prevent the respective laser beams from interfering with each other. However, with such an arrangement, if the photoconductive surface deviates from its original position even by a slight amount, a scanning line made through one laser scanning optical system and another scanning line made through another laser scanning optical system on the photoconductive drum will deviate from each other in the sub-scanning direction, and accordingly the former and latter scanning lines will not be combined with each other in the main scanning direction, so that a wide scanning line made by a combination of those scanning lines which extend in the main scanning direction across the photoconductive surface of the drum cannot be formed.